Photonic crystals

ABSTRACT

The invention relates to photonic crystals, i.e. three-dimensional dielectric structures which are opaque to electromagnetic radiation within a given wavelength irrespective of the direction of incidence. An inventive photonic crystal comprises, for instance, a matrix-type arrangement of free-standing dielectric micro-columns or cylinders having small diameters. Said micro-columns or cylinders are placed on a substrate whose thermal expansion coefficient between 60° C. and 85° C. is at least 50% lower than the thermal expansion coefficients of quartz glass, thereby enabling the thermal expansion of the substrate to be kept to a minimum even when high termperature variations occur; whereby the distance between the micro-columns supported by the substrate or the microcylinders of the photonic crystal varies only to a small amount, whereby the optical characteristics are essentially more stable than those of photonic crystals with a quartz glass substrate. Preferably, glass ceramics or, for example, Zerodur® are used as a substrate.

[0001] The present invention is directed to photonic crystals.

[0002] Photonic crystals are generally understood to be three-dimensional dielectric structures which are opaque to electromagnetic radiation within a specific wavelength range, regardless of the direction of incidence. In this context, the wavelength range is substantially determined by the arrangement, form, and size of the structures. One important type of photonic crystals is constituted of an evenly spaced, for example matrix-type, array of free-standing dielectric micro-columns or micro-cylinders having very small diameters and a substantial height relative thereto. For visible and infrared light, typical micro-cylinder diameters are in the range of one hundred to a few hundred nanometers.

[0003] To fabricate photonic crystals having micro-cylinders, a substrate is needed to support the micro-cylinders. For this, in known methods heretofore, inter alia, fused quartz is mostly used due to the mechanical and chemical stability it offers. In the following, it is understood that “photonic crystals” refer to the three-dimensional structure having micro-columns or micro-cylinders, along with the substrate that supports them. Because of their unusual properties, photonic crystals are excellently suited for manufacturing optical components, such as very narrow-band filters, modulable filters, add-drop filters, and integrated optical structures having 90′-deflection. Components of this kind are used in DWDM technology (dense wavelength division multiplexing) for transmitting data in optical waveguides, where electromagnetic radiation of various wavelengths is used as a carrier. At the present time, the channel separation between simultaneously used, adjacent channels is 0.4 nm, wavelengths in the range of about 1550 nm being used. In the future, to achieve a higher bandwidth for transmissions via optical waveguides, the channel separation will drop to 0.2 nm or less. Already now, but certainly even more so in the future, this requires components whose filter properties are extremely stable, particularly in response to substantial temperature fluctuations.

[0004] It is known, however that, under substantial temperature fluctuations, the optical properties of photonic crystals and, thus, in particular, also the transmission wavelength of filters based on photonic crystals can markedly change.

[0005] It is, therefore, the object of the present invention to provide photonic crystals, whose optical properties, in the technically relevant temperature range of between −60° C. and 85° C., are more stable than those of known photonic crystals having a fused quartz substrate.

[0006] This objective is achieved by photonic crystals having the features of claim 1.

[0007] A photonic crystal according to the present invention has a substrate whose thermal expansion coefficient, within the temperature range of between −60° C. and 85° C., is at least 50% less than the magnitude of the expansion coefficient of fused quartz. For that reason, even under conditions of substantial temperature fluctuations, the thermal expansion of the substrate is kept low; consequently, the pitch of the photonic crystal's micro-columns, i.e., micro-cylinders, supported by the substrate, also changes very little. For that reason as well, the optical properties of the photonic crystal also change very little, so that the optical properties are substantially more stable than those of photonic crystals having a fused-quartz substrate.

[0008] The thermal expansion coefficients of the substrate and/or of the fused quartz are, for example, linear thermal expansion coefficients.

[0009] Preferably, the magnitude of the thermal expansion coefficient of the substrate within the, preferably entire, temperature range of between −60° C. and 85° C. is at least 50% less than the magnitude of the thermal expansion coefficient of fused quartz.

[0010] Preferred is a substrate whose base is of a glass ceramic, whose thermal expansion coefficient within the temperature range of between −60° C. and 85° C. is less than 5×10⁻⁷/° C., less than 4×10⁻⁷/° C., less than 3×10⁻⁷/° C., less than 2×10⁻⁷/° C., very preferably less than 1×10⁻⁷/° C., and most preferably less than 5×10⁻⁸/° C. or less than 2×10⁻⁸/° C. However, a glass ceramic having an expansion coefficient within the last-mentioned two ranges is expensive to manufacture.

[0011] In principle, glass ceramics are composite materials having an amorphous glass phase and crystals intercalated therein. The crystals are produced by ceramization, i.e., controlled devitrification (crystallization), of glasses. They are produced by thermally treating a suitable glass in which crystals are produced as a result. For this, in production, suitable raw material is first melted, rectified, homogenized, and is then formed in a hot state. Following the cooling and annealing of the glass-like blank, a temperature treatment is carried out in which the crystallization takes place. During the temperature treatment, crystal nuclei form in the glass, and the crystals subsequently grow on these nuclei in response to a somewhat higher temperature.

[0012] The glass ceramic according to the present invention contains more than 50% by weight of SiO₂. Moreover, it may contain various loading materials, in particular, for example, calcium oxide (CaO), sodium oxide (Na2O) having, in part, larger quantities of boron trioxide (B2O3), aluminum oxide (A12O3), lead oxide (PbO), magnesium oxide (MgO), barium oxide (BaO) or potassium oxide (K2O). Since there is no need for the substrate to have any special optical properties, the influence the loading materials have on the optical properties may be disregarded when selecting the same. Glass ceramics are mechanically and chemically very stable, which is advantageous for the production of photonic crystals, but also for their later use.

[0013] Another subject matter of the present invention is a method for producing photonic crystals according to the present invention having the features of claim 8.

[0014] The present invention provides that, by properly selecting the substrate material which is based on SiO2, it is possible to also produce the photonic crystals of the present invention using all known methods for producing photonic crystals having fused-quartz substrates.

[0015] Therefore, the substantial benefit is derived that, to produce the photonic crystals in accordance with the present invention, there is no need to completely redevelop a method using, in principle, other types of steps, rather, in the development, one may build upon the known method.

[0016] For example, additive lithography may be used for the manufacturing. This method is explained in greater detail in SPIE vol. 2849 (1996), p. 248 ff., the content of this article being incorporated by reference, in its entirety, in the subject matter of the disclosure of this Specification.

[0017] Other preferred specific embodiments of the present invention are elucidated in the description that follows and in the dependent claims.

[0018] The glass ceramic preferably has a glass phase and a crystalline phase distributed therein having a thermal expansion coefficient which, within the temperature range of between −60° C. and 85° C., is less than that of the glass phase. Quite preferably, a glass ceramic is used whose crystalline phase, at least in parts of the temperature range between −60° C. and 85° C., has a negative thermal expansion coefficient. Since the thermal expansion coefficient of a composite material depends to a great degree, inter alia, on the thermal expansion coefficients of its components, the result is an especially low thermal expansion coefficient of the glass ceramic.

[0019] The weight proportion of the crystalline phase also influences the thermal expansion coefficient of the glass ceramic. For that reason, when the thermal expansion coefficient of the crystalline phase is less than that of the glass phase, in particular when it is negative, the proportion of the crystalline phase is preferably selected to be so high that, within the temperature range of between −60° C. and 85° C., the thermal expansion is especially low. The weight proportion of the crystalline phase in the glass ceramic is preferably between 60 and 90%, quite preferably between 70 and 80%.

[0020] To ensure that the thermal expansion in the glass ceramic is approximately equivalent in all locations, in order to substantially avoid thermal stresses, the crystalline phase is preferably distributed substantially homogeneously in the glass ceramic.

[0021] Particularly suited for manufacturing the photonic crystals according to the present invention and, thus, preferred is the glass ceramic ZERODUR® of the firm Schott (Mainz, Germany), which has a very low thermal expansion coefficient of less than 10⁻⁷/° C.=0, 1 per million/° C. within the temperature range of between −60° C. and 85° C. and a very homogeneous and fine distribution of the crystalline phase. These properties facilitate the processing of the substrate and make attainable excellent properties of the same.

[0022] The present invention is elucidated in the following on the basis of preferred specific embodiments and with reference to the drawing.

BRIEF DESCRIPTIONS OF THE INVENTION

[0023] The figures show:

[0024]FIG. 1 a schematic perspective illustration of a detail of a preferred specific embodiment of the photonic crystal according to the present invention; and

[0025]FIG. 2 a plan view of a detail of the photonic crystal of FIG. 1.

[0026]FIG. 1 shows photonic crystal 1 according to the present invention which was produced using additive lithography, including the electron beam-induced deposition of a three-dimensional structure 2 on a substrate 3. For the production, a beam current of 100 pA to 200 pA was used.

[0027] Three-dimensional structure 2 is arranged on substrate 3 and adheres thereto. The deposited material is a composition of essentially spherical, metallic nano-crystals in a matrix of insulating amorphous carbon or polymer, grown from CH_(x) radicals. For example, platinum and/or gold are used as metals.

[0028] The exemplary three-dimensional structure 2 includes a multiplicity of essentially cylindrical micro-columns or rods 4 of Me₂Au arranged in a two-dimensional, evenly spaced matrix. The energy band gap is 1.38 eV, which corresponds to a photon wavelength of 900 nm. Length h of the rods is about 2000 nm. Structure 2 is, respectively rods 4 are, nanocrystalline or amorphous.

[0029] With reference to FIG. 2, diameter d of rods 4 is about 140 nm and pitch a of rods 4 within the evenly spaced matrix-shaped array in the horizontal and vertical direction is, in each instance, 320 nm from the center to center of adjacent rods 4.

[0030] Variations in the dimensions, e.g., a diameter of the rods within the range of 80 nm to 300 nm, are likewise producible and/or usable. Likewise, other forms are producible and/or usable as cylindrical rods.

[0031] Alternative methods for manufacturing the photonic crystals include:

[0032] chemically assisted ion-beam etching;

[0033] X-ray lithography and forming;

[0034] optical lithographic bonding (so-called on-top bonding);

[0035] three-dimensional laser deposition or

[0036] electron-beam nano-lithography.

[0037] It is especially advantageous to use a substrate, in particular a glass ceramic, which includes or is made of micro-crystals of a preferably average size within the range of about 50 nm. However, micro-crystal sizes other than those mentioned, e.g., smaller or larger than 50 nm, are likewise usable within the framework of the present invention.

[0038] In addition, a substrate 2 is preferably used which, chemically, mechanically, and/or in terms of polishing characteristics, is substantially similar or even identical to the fused quartz. Substrate 2 has a smooth, e.g., polished surface, which is especially beneficial when combined with the three-dimensional structures of the photonic crystal according to the present invention. 

What is claimed is:
 1. A photonic crystal, characterized by a substrate whose thermal expansion coefficient, within the temperature range of between −60° C. and 85° C., is at most half as great as the magnitude of the thermal expansion coefficient of fused quartz.
 2. The photonic crystal as recited in claim 1, characterized by a substrate of glass ceramic, whose thermal expansion coefficient within the temperature range of between −60° C. and 85° C. is less than 5×10⁻⁷/° C.
 3. The photonic crystal as recited in claim 2, wherein the glass ceramic has a glass phase and a crystalline phase distributed therein having a thermal expansion coefficient which, within the temperature range of between −60° C. and 85° C., is less than that of the glass phase.
 4. The photonic crystal as recited in claim 3, wherein the thermal expansion coefficient of the crystalline phase is negative, at least in parts of the temperature range between −60° C. and 85° C.
 5. The photonic crystal as recited in claim 4, wherein the weight proportion of the crystalline phase in the glass ceramic is between 60 and 90%.
 6. The photonic crystal as recited in claim 5, wherein the weight proportion of the crystalline phase in the glass ceramic is between 70 and 80%.
 7. The photonic crystal as recited in one of the preceding claims, wherein the crystalline phase is distributed homogeneously in the glass ceramic.
 8. A method for manufacturing photonic crystals, wherein as a substrate for the crystals, a substrate of glass ceramic is used, whose thermal expansion coefficient within the temperature range of between −60° C. and 85° C. is less than 10⁻⁷/° C.
 9. The method as recited in claim 8, wherein the glass ceramic has a glass phase and a crystalline phase distributed therein having a thermal expansion coefficient which is less than that of the glass phase.
 10. The method as recited in claim 9, wherein the thermal expansion coefficient of the crystalline phase is negative.
 11. The method as recited in claim 10, wherein the weight proportion of the crystalline phase in the glass ceramic is between 60 and 90%.
 12. The method as recited in claim 11, wherein the weight proportion of the crystalline phase in the glass ceramic is between 70 and 80%.
 13. The method as recited in one of claims 8 through 12, wherein the crystalline phase is distributed homogeneously in the glass ceramic. 